Belt-type continuously variable transmissions have been well-known and put into practical use in automobiles and other vehicles. Such belt-type continuously variable transmission comprises, for example, drive and driven pulleys each having a variable pulley width, and a metallic V-belt trained therearound. The speed ratio of the transmission is controlled by controlling the adjustment of the pulley widths of the drive and driven pulleys. For the controlled adjustment of the pulley widths, the axially movable member of the halves comprising each pulley is provided with a hydraulic cylinder, into which a pulley thrust control pressure is supplied. Control of the pulley thrust pressures into the respective cylinders of the drive and driven pulleys allows varying the pitch diameters of the pulleys, around which the V-belt runs, and thereby controls the speed ratio of the transmission.
As a means for controlling the supply of pulley thrust pressures into the respective cylinders, a shift valve (four-way valve) is provided, and through the shift valve, high and low pulley thrust control pressures PL and PH are supplied to the drive and driven pulleys, respectively or vise versa. The low pulley thrust control pressure PL is the lowest possible pressure which allows torque transmission without any slippage between the pulleys and the belt, and the high pulley thrust control pressure PH is a pressure which works to control the speed ratio of the transmission and is greater than the low pulley thrust control pressure PL.
FIG. 9 shows one such Prior art shift valve. This shift valve 153 comprises a high pulley thrust pressure port 153b to receive the high pulley thrust control pressure PH; low pulley thrust pressure ports 153c provided on both sides of the high pulley thrust pressure port 153b to receive the low pulley thrust control pressure PL; a drive port 153d connected to the cylinder of the drive pulley (hereinafter referred to as "drive cylinder") of a continuously variable transmission; a driven port 153e connected to the cylinder of the driven pulley (hereinafter referred to as "driven cylinder"); and a spool 153a, whose position is controllable from a neutral position either to the right or to the left. When the spool 153a is controlled positionally, the land portions thereof function to interconnect or block the ports. The right end of the spool 153a is pushed by a spring or a hydraulic pressure, and the left end is pushed by a shift control pressure P.sub.SV provided by, for example, a current-controlled solenoid valve, balancing the biasing force of the spring so as to control the position of the spool 153a.
As shown in FIG. 9, when the spool 153a of this shift valve 153 is at a neutral position, the low pulley thrust pressure ports 153c are connected to the drive and driven ports 153d and 153e respectively through the openings present due to underlap (L1-L2) of the land portions of the spool 153a and the drive and driven ports 153d and 153e. Therefore, the low pulley thrust control pressure PL is supplied sufficiently into both drive and driven cylinders. As a result, the pulley thrust pressures of the drive and driven pulleys are retained at the low pulley thrust control pressure PL, which is a pressure necessary to avoid slippage of the belt, while the speed ratio is not being controlled to change.
When the spool 153a is shifted either to the right or to the left from the neutral position, the low and high pulley thrust control pressures PL and PH are supplied selectively into the respective cylinders, each pressure with a flow proportional to the cross-sectional area of the interconnection of respective ports. As a result, changes of the pitch diameters of the pulleys, which brings a change in the speed ratio of the transmission, are effected. FIG. 10 shows the pressures inside the drive and driven cylinders with respect to the position of the spool 153a.
However, the shift valve 153, with the above described construction, presents a problem that the pressure inside one of the cylinders can decrease to a pressure below the low pulley thrust control pressure PL (refer to the portions indicated by "A" in FIG. 10). The reason is that, at one of the ports, the opening from the above mentioned underlap substantially disappears when the spool 153a is shifted a little either to the right or to the left, creating reductions in the flows from both high and low pulley thrust pressure ports 153b and 153c to one of the drive and driven ports 153d and 153e whose opening is narrowing. When this happens, the belt of the transmission slips over the pulley whose pressure has decreased, and torque transmission is impaired.
In order to prevent such pressure drop, the underlap between the land portions and the drive and driven ports may be made larger. However, if the shift valve were constructed as such, part of the high pulley thrust control pressure PH flowing into the shift valve from the high pulley thrust pressure port 153b would flow into the low pulley thrust pressure port 153c, as shown in FIG. 11, thereby increasing the load of the oil pump, which works as pressure source for producing each control pressure. The result would be a new problem. Furthermore, such construction would present another problem of reduced controllability for the variable transmission. The reason is that such construction would widen the insensitive range or dead zone (refer to FIG. 10), where the pressure in each cylinder is not changed while the spool 153a is being shifted.
It is also possible to set each control pressure a little higher so as to keep the belt running without slippage even when the pressure inside one of the cylinders decreases to a pressure a little below the low pulley thrust control pressure PL. However, this would be no different from the above mentioned underlap increase with respect to presenting a problem of increased load for the oil pump.
Furthermore, the speed ratio, i.e., ratio of the pitch diameters of the pulleys, of a belt-type continuously variable transmission is prone to be affected by changes in the torque transmitted by the belt. In order to perform a steady control of the speed ratio, it is necessary to carry out positional control of the spool, i.e., control of the control pressures, accurately in consideration of the relationship between transmission torque and pressure-to-flow rate characteristics, and to carry out feedback control of the speed ratio. All of these contribute to complicate and burden the control operation involved with the shift valve of the prior art.
In Japanese Patent Laid-open Publication No. 62-196448, an apparatus is proposed which supplements a low pulley thrust control pressure when the pressure inside one of the cylinders decreases below the desired low pulley thrust control pressure. The low pulley thrust control pressure is supplemented directly into the cylinder whose pressure has decreased, without making it flow through the shift valve. However, this apparatus presents a problem that a constant pressure cannot be applied continuously in each cylinder unless control of the spool in the shift valve, i.e., control of the thrust of the solenoid pushing the spool, is performed with substantially high precision.